This invention relates to a block deformation removing filter, particularly to a block deformation (or blocking noise) removing filter which is used to remove a block deformation (or blocking noise) taking place when for example an image is encoded by compression and the encoded image signal is decoded.
When an image is encoded by compression with high efficiency by way of an image compression coding technique, it may often present an image degradation associated with various compression coding techniques. For example, when an image is encoded by compression in units of block by way of the discrete cosine transformation (DCT), the block boundaries may become discontinuous to each another, thus resulting in block deformation.
The possible block deformation is most outstanding image quality degradation in a visual sense when a compression coding technique is applied in units of block, so that the improvement of the image quality requires minimization or removal of such deformation or discontinuity. So far in order to solve this problem, a decoded image involving a block deformation is filtered by a low-pass filter locally or in the large.
FIGS. 1A and 1B are illustrations showing how to remove the block deformation by way of the low-pass filter. FIG. 1A shows intensity values of decoded image before filtering, while FIG. 1B after filtering. In this connection, FIGS. 1A and 1B respectively show four scanning lines as representation which cross a block boundary in the image at right angle, and the heights of respective lines show intensity levels or values (shown by solid circles) of pixels lined up on each line.
FIG. 1A shows differentials A in intensity level taking place on every line crossing at right angle with the block boundary shown by a dotted line, where the differentials may be slightly different but essentially same, which cause a discontinuous block boundary resulting in a block deformation. Such a block deformation contributes to assume a mosaic pattern of a decoded image in those points, thus causing a significant visual difficulty.
Therefore, as shown in FIG. 1B, possible differentials xcex94 in the intensity level at the block boundary are smoothed by way of low-pass filter, thereby taking the straight edge off in intensity value, resulting in a minimized block deformation.
However, the process of block deformation removal by way of low-pass filter also removes a necessary high frequency signal component of an original image other than the block deformation, thus causing a blurred image.
Further, when the differentials xcex94 in the block boundary shown in FIG. 1A are small, the effect acquired by taking off the straight edge shown by the differentials xcex94 can be very slight than expected, thus resulting in an inadvertent failure of effectively minimizing block deformation.
For example, when the differentials xcex94 equal xe2x80x9c1xe2x80x9d in a 256-level image, the pixel value cannot have fractional value so that the favorable effect of low-pass filter can not be expected at all, thereby resulting in a clear visual recognition of block deformation. On the other hand, even a block deformation with such very small differential xcex94 would be often visible to the eye, so that the filtering by way of low-pass filter which cannot remove a block deformation with a small differential xcex94 has failed to successfully suppress image quality degradation due to the block deformation.
To solve the above problems, a purpose of the present invention is to provide a block deformation (or blocking noise) removing filter and its removing method which can not only remove block deformation without affecting the high frequency signal component in an original image, but also successfully remove even block deformation with very small differential value of the signal in the block boundary.
The present invention provides a block deformation removing filter comprising: a detector responsive to pixel signals included in a plurality of pixel blocks forming an image to detect a difference between at least two pixel signal levels in the vicinity of a block boundary of the pixel blocks; a comparator to compare the difference with a threshold value; and an adder to add values to the pixel signal levels in the vicinity of block boundary in accordance with the result of the comparison.
Further, present invention provides a method of filtering an image signal, comprising the steps of: supplying pixel signals included in a plurality of pixel blocks forming an image; detecting a difference between at least two pixel signal levels in the vicinity of block boundary of the pixel blocks; comparing the difference with a threshold value; and adding values to the pixel signal levels in the vicinity of block boundary in accordance with the comparison result.
Further, the present invention provides an image processing apparatus comprising: scanning means for scanning pixel signals included in a plurality of pixel blocks forming an image in a first scanning direction and next in a second scanning direction orthogonal to the first direction with respect to the image; and removing means responsive to the pixel signals scanned in the first or the second direction for removing deformation of the pixel blocks in the first or the second direction, wherein the pixel signals deformation-removed in the first direction are scanned by the scanning means in the second direction, the pixel signals scanned in the second direction being processed by the removing means.
Further, the present invention provides a method of processing an image comprising: scanning pixel signals included in a plurality of pixel blocks forming an image in a first scanning direction with respect to the image; removing deformation of the pixel blocks in the first direction in response the pixel signals scanned in the first direction; scanning the pixel signals deformation-removed in the first direction in a second direction orthogonal to the first direction with respect to the image; and removing deformation of the pixel blocks in the second direction in response the pixel signals scanned in the second direction.
Further, the present invention provides an image processing apparatus comprising: a decoder to decode pixel signals included in a plurality of pixel blocks forming an encoded image; a detector responsive to the decoded pixel signals to detect a difference between at least two pixel signal levels in the vicinity of block boundary of the pixel blocks; a comparator to compare the difference with a threshold value; and an adder to add values to the pixel signal levels in the vicinity of block boundary in accordance with the comparison result.
Further, an image processing apparatus comprising: an encoder to encode predictive error signals of pixel signals included in a plurality of pixel blocks forming an image; a decoder to decode the encoded pixel signals; a detector responsive to the decoded pixel signals to detect a difference between at least two pixel signal levels in the vicinity of block boundary of the pixel blocks; a comparator to compare the difference with a threshold value; an adder to add values to the pixel signal levels in the vicinity of block boundary in accordance with the comparison result; a predictor to generate predictive signals based on output signals of the adder; and a differentiator to calculate differentials of the predictive signals and the pixel signals included in the pixel blocks forming the image.
Further, an image processing apparatus comprising: a decoder to decode pixel signals included in a plurality of pixel blocks forming an encoded image; an adder to add predictive signals to the decoded pixel signals; a detector responsive to output signals of the adder to detect a difference between at least two pixel signal levels in the vicinity of block boundary of the pixel blocks; a comparator to compare the difference with a threshold value; an adder to add values to the pixel signal levels in the vicinity of block boundary in accordance with the comparison result; and a predictor to generate the predictive signals based on output signals of the adder.